The aim of the present investigation is to study the influence of H2SO4 concentration on the electrochemical activity, the phase composition and the structure and morphology of the PbO2 particles. The study is performed through cycling (between 700 and 1600mV versus Hg/Hg2SO4 electrode) of a Pb/PbO2/PbSO4 electrode immersed in sulfuric acid solutions of various concentrations (ranging within 2 orders of magnitude: 6.0-0.05 M H2SO4). In this concentration region, sulfuric acid dissociates in two steps resulting in the formation of HSO4- and SO42- ions, respectively. It has been established experimentally that the electrochemical activity of the PbO2/PbSO4 electrode depends on the concentration of HSO4- ions in the solution. Three acid concentration regions can be distinguished: (a) active acid concentration region (5.0M > C-H2SO4 > 0.5M), where the concentration of HSO4- ions is the highest and a betaPbO(2) phase is formed; PbO2) particles are drop-like in shape and contain large hydrated (gel) zones; the electrode has the highest capacity; (b) passive high concentration region (C-H2SO4 > 5.0 M), where the concentration of HSO4- ions decreases at the expense of formation of H2SO4 molecules; crystal-shaped alphaPbO(2) particles are formed; the capacity of the electrode declines; (c) passive low concentration region (C-H2SO4 < 0.5 M), where the concentration of HSO4- ions decreases at the expense of the formation of SO42- ions; the content of alphaPbO(2) in the anodic layer increases; PbO2 particles are crystal-shaped and are interconnected in dendrites; the capacity of the electrode declines. The above electrochemical behaviour of the PbO2/PbSO4 electrode is explained by the mechanism of the reactions in the gel zones of the PbO2 Particles and by the influence of HSO4- ions on the number of electrochemically active particles. On grounds of the obtained experimental results it has been established that the working interval within which the C-H2SO4 may change on cycling is from 5.0 to 1.5 M, i.e. 3.5 M H2SO4 per 11 of H2SO4 solution with s.g. 1.28 takes part in the reactions on both battery plates. This is the maximum amount of H2SO4 in the solution that would have no detrimental effect on the positive plates of the lead-acid battery. (C) 2004 Elsevier B.V. All rights reserved.